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I just checked the RecordValue object which you have available within the script and this object gives you access to children of the current record, and also to it's parent.
The problem here is, that in case you need to access a sibling record, and the current record is already at the root level, the getParent method will not return you anything (as there is no parent) and there is no getSibling method (yet).
You can not use the getFieldValue method as the validation is executed within the UI before the characteristic values have been applied to the detail model of the item, thus it's not even in there.
Please open a bug issue for this, we need to extend the recordValue object a little bit so you can access the sibling records.